LibreOffice installation process is currently designed to:
- Collect information about currently installed features (if present).
- Ask user to refine the selected features.
- Run normal uninstall of previous LibreOffice (if present).
- Install the selection.
So in general, it is preferable to install LibreOffice over existing version, to automatically keep the same set of components without need to select them again (where user has a non-standard set). In all cases, the old version will be uninstalled prior to installation of new version, the same way as would happen manually. (By the way, this is somewhat unfortunate, because it slows down the installation process, removes pinned items, and disallows to rollback to previous working state when uninstallation of old version has succeeded, but new version failed to install for some reason; but that is not related to the uninstallation/installation process being joined; the same would happen if the two be separated, and this unfortunate operation is also fortunate in other sense, because the concerns about “it’s safer to manually uninstall older than install newer” are unwarranted).
No installation/uninstallation process touches any user profiles: they are created/updated by LibreOffice process (when you run the installed program), not by installer.
In cases where something wrong happens (MSI errors), there’s a FAQ that lists some of available options; and one of them is “Use troubleshooter tool from Microsoft”, which is the tool MS has created to fix numerous errors that happen during the work of the system service. Note that that’s a general-purpose tool, existing due to the inherent flaws of the Windows Installer service, and not specific to LibreOffice installer.
About “staying with the previous bitness”, which @Cookievore suggested: I don’t think it actually has solid foundation. If you have 64-bit system, you likely would be better served by 64-bit LibreOffice; so the choice is more based on your system architecture, not on previous LibreOffice architecture. There are rare cases where your choice of LibreOffice architecture was dictated by e.g. availability of ODBC drivers for the platform; but in that case, you likely clearly know what you need and why.
All that relates to the case when you have a failure during installation, to try to let you see the picture and find a solution. If your problem is that you are trying to update using automatic updater, then, as @Cookievore mentions, it only notifies about existing updates, and user needs to download and launch the installer manually.